The present invention relates to a process for the extrusion of thermoplastic compositions and more especially to a process for the extrusion of foamed thermoplastic compositions. The process disclosed herein is useful for extruding compositions comprising a major portion of at least one thermoplastic resin which is either amorphous or crystalline in nature. The process is characterized by increased throughput rates and higher quality extruded products.
In the conventional process employed in the plastics industry for extruding thermoplastic compositions, beads or pellets of at least one thermoplastic resin and various other additives are introduced into the feed zone of a screw-type extruder. In the extruder, the thermoplastic resin and additives are heated and mixed to form a substantially homogenous, continuous, flowable composition which is then forced by the screw through an extrusion die to produce a product of the desired shape and dimensions.
As the thermoplastic composition passes through the extruder, its temperature increases significantly due to the combined shear and compressive forces applied to the material by the rotating extruder screw. For a given extruder, the magnitude of the temperature increase varies according to the rotation rate of the extruder screw and the shear properties of the particular thermoplastic composition being run. While some heating is desirable and necessary for achieving satisfactory extrusion, excess heat must be removed from the material downstream of the extruder in order to retain the shape and integrity of the extruded product. Typically, this is done by passing the extrudate over chill rolls or through cooling vats downstream of the extrusion die.
Because the temperature of the extrudate exiting the extrusion die is proportional to the rotation rate of the extruder screw when operating under standard conditions (i.e., an increase in throughput requires a higher temperature), conventional extrusion lines have been limited as regards their throughput rates by the capacity of the cooling equipment downstream of the extrusion die. Even where the downstream cooling capacity is adequate, the extrudate can undergo thermal shock if its temperature is reduced too rapidly over a wide temperature differential, thereby adversely affecting its mechanical properties.
Particular problems are encountered in the extrusion of foamed thermoplastic compositions. Extruders of foamed thermoplastic compositions are typically run at high pressures to keep the blowing agent in the polymer condensed until the composition emerges from the extrusion die. If the temperature of the foamed product as it emerges from the extrusion die is significantly greater than that required to achieve satisfactory extrusion, the blowing agent will overexpand once the pressure is relieved, resulting in cell rupture and the loss of dimensional stability and compositional integrity. If the temperature is too low, expansion will be incomplete and poor density properties will result. For some polymers, such as polyethylene, the correct temperature window is only about .+-.2.degree. F.
Furthermore, the problem is not only one of achieving a specific absolute temperature, but also of achieving uniformity of temperature. If temperature gradients exist within the polymer mass, uneven blowing takes place, again causing ruptured cells and poor density values. At high throughputs, the existence of temperature gradients is more likely to occur.
Therefore, in connection with the extrusion of foam products, it is extremely difficult to obtain an increase in throughput for an extrusion line while at the same time not causing a deterioration in the physical properties of the resulting product, such as the size, uniformity and integrity of the cells and the density value of the foamed polymer. In addition, these problems are exacerbated when, as often desired, various additives are incorporated into the foamed product, such as, for example, a fire-retardant.
Several measures have been taken in the past to solve these problems. For example, it is common to employ two separate extruder screws connected in series. See, e.g., U.S. Pat. No. 3,860,220. In this configuration the screw of the second extruder merely acts as an auger to convey the thermoplastic composition through the extruder, which is jacketed and cooled with a circulating cooling medium. However, the use of a second extruder in this capacity has proven to be very expensive, both from an equipment and an energy standpoint; and it has been found to be an inefficient method for cooling a foamed material. Temperature gradients are actually produced in a second screw, because heat is generated at the screw, while cooling is applied from the outside. Furthermore, because of the high pressures employed in foam extrusion, problems are often encountered with the rear seals of the second extruder screw. Failure of the rear seals can result in damage to the gear box from the escaping polymer as well as undesirable leakage of the blowing agent.
Another solution is to decrease the rotational speed of the extruder screw; however, this measure is obviously antithetical to an increase in extrusion line throughput.
Other measures have included the inclusion of cooling devices in the downstream portion of the extruder (See, e.g., U.S. Pat. Nos. 3,385,917, 3,151,192, 3,444,283 and 3,658,973 and British Pat. No. 2,003,080) or in conjunction with the extrusion die itself (See. e.g., U.S. Pat. Nos. 3,393,427 and 4,088,434 and U.S.S.R. Pat. No. 592,610). These die units are initially very expensive and even more expensive to modify in this manner. Furthermore, they are not effective heat exchange elements, and therefore do not permit significant increases in throughput.
It is also possible to increase the amount of cooling capacity downstream of the extrusion die. See, e.g., U.S. Pat. No. 3,764,642. However, this gives rise to the problem of thermal shock, mentioned above, and moreover, the most essential cooling often is required upstream of the die orifice in order that the resin can be extruded within a certain required temperature range. This is essential in the case of foam extrusion.
Other attempts have been made to interpose some sort of a cooling device between the extruder and the extrusion die. See, e.g., U.S. Pat. Nos. 3,310,617, 3,275,731, 3,751,377, 3,588,955, 3,827,841 and 3,830,901. These efforts have indeed increased the total heat exchange or cooling capacity of the extrusion line; however, they have not been successful in solving the problem of temperature uniformity, as evidenced, for example, by the need to include an additional mixing device downstream of the heat exchange or cooling device, e.g., in U.S. Pat. No. 3,588,955, FIG. 3. Furthermore, while some increase in throughput has been accomplished by these measures (See, e.g., U.S. Pat. No. 3,827,841), it has not been possible to achieve such increases above a certain level, while at the same time producing a foamed product having the desired physical properties.
An extrusion process for thermoplastic composition has therefore been needed which would simultaneously permit increased throughput in an extruder and not result in deterioration of the physical properties of the extruded product. A process was particularly needed which would permit the extrusion of foamed thermoplastic compositions at increased production rates within narrow temperature limits and with excellent physical properties, preferably employing an extruder having a single screw.